The modem information revolution has led to an ever increasing demand for data storage systems. As a case in point, CD and DVD disks represent successful high volume data storage technologies. One major advantage of these technologies is that reading or writing of data is accomplished by shining light on the disk so there is no physical contact between the media and the optical head. However, the total storage capacity of these disks is limited by the size of the smallest marks on the surface of the media that can be read by the wavelength of light employed. Many attempts have been made to develop data storage systems with progressively smaller marks. However, the required equipment is prohibitively expensive, and the data access rates tend to be unacceptably slow.
One way to increase the storage capacity of a medium is to record the information depthwise, rather than just on the surface. There could be used holography, two-photon optics, and similar methods for illuminating media in three dimensions, with the goal of producing marks in three dimensions, and thereby providing very high data capacity systems.
Bleaching and photoreactions (e.g., photochromicity) of organic dyes have also been used as a means to record optical data, both in a single layer in writeable CD-type media, and depthwise (dissolved in a bulk piece of polymer). However, a large amount of optical power is required in these systems to produce readable marks, therefore the rate of recording of such media is slow. Also, many photochromic systems also tend to fade over time.
Holographic recording has also been achieved by optically induced birefringence in suitable polymers, a process which relies on photo-alignment of the side chains within the polymers. Once again, a large amount of optical power is required, and this process is inefficient and slow. In addition, the fidelity of the recorded information may degrade with time since optically induced orientation tends to relax over time in polymers.
JP 2000-086588 discloses a recording medium using changes in circular dichroism based on the interconversion of chiral norbornadiene and quadricyclane derivatives. However, this technique requires enantiomerically enriched compounds that are difficult to synthesize. Furthermore, this application does not disclose the use of sensitizers for photoinduced electron transfer.
U.S. Pat. No. 5,759,721 discloses a holographic recording medium which uses a photopolymerization technique which can also be used for recording information optically in three dimensions. There is a problem with this process, however, in that photopolymerization is usually accompanied by shrinkage of the material which is a consequence of the process of forming new chemical bonds among the constituents. Any dimensional changes that occur on writing limit the resolution that can be achieved, and reduce the data capacity of the medium. In addition, photopolymerization generally requires the use of low molecular weight reactants so that media made from these materials tend to be undesirably soft or sticky. Furthermore, the most common method of photopolymerization, free radical polymerization, is subject to interference by atmospheric oxygen which causes undesirable inconsistencies in the process.
It is an object of this invention to increase the storage capacity of a optical recording material. It is another object of this invention to provide an optical recording material which can record information depthwise, rather than just on the surface. It is still another object of this invention to provide an optical recording material which does not substantially change dimensions upon recording.